Effects of Ni concentration and aging conditions on multistage martensitic transformation in aged Ni-rich Ti-Ni alloys

The effects of Ni concentration and aging conditions on the multistage martensitic transformation (MMT) in aged Ni-rich Ti-Ni alloys have been investigated by differential scanning calorimetry and in situ scanning electron microscopy. The effect of Ni concentration was evaluated using Ti-50.6, 50.8 and 51.0 at.% Ni alloys. These alloys were heat treated at 1223 K for 3.6 ks and then aged at 773 K for 3.6 ks. Although the triple-stage transformation appeared in the Ti-50.6 and 51 at.% Ni alloys during cooling, the transformation sequence of the two alloys was completely different. Quadruple-stage transformation was observed in the Ti-50.8 at.% Ni alloy. The characteristic microstructure responsible for the MMT in each of the aged alloys strongly depended on the degree of Ni supersaturation and the aging temperature and time. We have proposed and experimentally verified a general rule that explains the effects of Ni concentration and aging conditions on the microstructural changes and thus the MMT sequences. This will allow the MMT in Ni-rich Ti-Ni alloys to be controlled by selecting appropriate aging conditions.